Flame-Surface Advection in Transient Periodic Flow

Aldredge, R.C.

doi:10.1080/00102202.2014.967397

Cited by 4 publications

(2 citation statements)

References 28 publications

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“…In premixed turbulent combustion, burning velocity is enhanced linearly by low-intensity turbulence. For a better understanding of the relationship between flame surface advection and the intensity of the excitation flow, Aldredge [46] studied the advection of a flame front in a transient periodic flow (spatially and temporally) wherein different levels of turbulent intensity (low, moderate and large) of flow perturbations were considered analytically and numerically. The results were consistent with those obtained in the earlier investigations [44].…”

Section: The Effect Of Flow Oscillation On Turbulent Combustionmentioning

confidence: 99%

The Effect of Unsteady Stretch on Laminar Premixed Curved Flames

Sahafzadeh¹

2023

Preprint

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Laminar flamelets are often used to model premixed turbulent combustion. The libraries of rates of conversion from chemical to thermal enthalpies used for flamelets are typically based on counter-flow, strained laminar planar flames under steady conditions. The significance of transient strain has been discussed in the literature with most assertions being that their chemical time scales are sufficiently short compared to the turbulent time scales to treat them as quasi-steady. Less discussed is the unsteady motion of a curved flame front component of stretch rate. This thesis seeks further understanding of the effect of stretch rate on premixed flames by developing and validating a model for use with transient premixed laminar flame dynamics in a cylindrically-symmetric outward radial flow geometry (i.e., inwardly propagating flame). A FORTRAN code is developed and validated which models a laminar premixed flame exposed to an oscillating mass flowrate. This code solves transient equations of continuity, momentum, energy, and individual species in radial coordinates. In this model, flame response is studied when the flow and scalar fields remain aligned (i.e., no strain). The model is applied to conditions in which the flame expands (positive stretch) and contracts (negative stretch) radially by the addition of the externally-defined oscillating mass flow rate. The transient response of laminar premixed flames results in amplitude decrease and phase shift increase with increasing frequency. In order to implement the transient behaviour of flamelets in turbulent modelling more efficiently, a frequency response analysis is applied as a process characterization tool to simplify the complex non-linear behaviour using flame transfer functions. It is shown that with increasing frequency of the perturbation, when equivalence ratio is kept constant, or with decreasing equivalence ratio in the same frequency, non-linear behaviour of the flame becomes prominent. Therefore, linear models can only predict the flame behaviour with accuracy below the threshold of when the fluid and chemistry time scales are the same order of magnitude. Various nonlinear models are studied in order to find the most appropriate flame transfer function for higher frequencies to extend the predictive capabilities of these models.

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Section: The Effect Of Flow Oscillation On Turbulent Combustionmentioning

confidence: 99%

The Effect of Unsteady Stretch on Laminar Premixed Curved Flames

Sahafzadeh¹

2023

Preprint

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“…It was concluded that at least part of the "bending effect," which is normally seen in premixed combustion occurring in high-intensity turbulence [5], results from local flame stretch modifying the local consumption speed. For a better understanding of the relationship between flame surface advection and the intensity of flow excitation, Aldredge [6] studied the advection of a flame front in a transient periodic flow (spatially and temporally) where different levels of turbulent intensity of flow perturbations were considered analytically and numerically. In recent research, Aldredge [7], has studied the flame surface and burning rate increase due to wrinkling by analyzing the effect of multiscale periodic transient flow on isothermal-flame propagation.…”

Section: Introductionmentioning

confidence: 99%

Predicting the consumption speed of a premixed flame subjected to unsteady stretch rates

Sahafzadeh

Dworkin

Kostiuk

2018

Combustion and Flame

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The stretched laminar flame model provides a convenient approach to embed realistic chemical kinetics when simulating turbulent premixed flames. When positive-only periodic strain rates are applied to a laminar flame there is a notable phase lag and diminished amplitude in heat release rate. Similar results have being observed with respect to the other component of stretch rate, namely the unsteady motion of a curved flame when the stretch rates are periodic about zero. Both cases showed that the heat release rate or consumption speed of these laminar-premixed flames vary significantly from the quasi-steady flamelet model. Deviation from quasi-steady behaviour increases as the unsteady flow time scale approaches the chemical time scale that is set by the stoichiometry. A challenge remains in how to use such results predictively for local and instantaneous consumption speed for small segments of turbulent flames where their unsteady stretch history is not periodic. This paper uses a frequency response analysis as a characterization tool to simplify the complex nonlinear behaviour of premixed methane air flames for equivalence ratios from 1.0 down to 0.7, and frequencies from quasi-steady up to 2000 Hz using flame transfer functions. Various linear and nonlinear models were used to identify appropriate flame transfer functions for low and higher frequency regimes, as well as extend the predictive capabilities of these models. Linear models were only able to accurately predict the flame behaviour below a threshold of when the fluid and chemistry time scales are the same order of magnitude. Other proposed transfer functions were tested against arbitrary multi-frequency stretch inputs and were shown to be effective over the full range of frequencies.

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Flame propagation in multiscale transient periodic flow

Aldredge

2017

Combustion and Flame

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Flame-Surface Advection in Transient Periodic Flow

Cited by 4 publications

References 28 publications

The Effect of Unsteady Stretch on Laminar Premixed Curved Flames

The Effect of Unsteady Stretch on Laminar Premixed Curved Flames

Predicting the consumption speed of a premixed flame subjected to unsteady stretch rates

Flame propagation in multiscale transient periodic flow

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